This invention is an improvement on the station set interface circuits (referred to here generally as telephone subscriber interface devices) described in Canadian Patent No. 1,202,740 to Cruder et al (referred to here as "Cruder") and U.S. Pat. No. 4,219,700 to Panizzon et al (referred to here as "Panizzon"). Both describe telephone subscriber interface devices which include the following features.
1. Privacy. When another party on the party line is off-hook, the subscriber receives a busy tone and is disconnected from the telephone line (see Cruder, page 10 and Panizzon, column 9). PA1 2. Means for breaking-in. On a subscriber going off-hook, while other parties are using the party line, the subscriber receives a busy tone indicating that the telephone line is in use. The subscriber may break-in by manually hanging up briefly (hook-flash). While the subscriber is connected to the line, the other parties receive a signal indicating the invasion of privacy (Cruder page 11, Panizzon column 9). PA1 3. Means for making calls to parties on the same party line (revertive calls). (See Cruder, page 12 and Panizzon, columns 11 and 12). PA1 4. Means for terminating a revertive call that is not completed (Cruder, page 13). PA1 5. Means for detecting and decoding coded ringing signals for the subscriber station without there being audible ringing at the subscriber station for every call to other parties on the party line. (Cruder, page 16 and Panizzon, columns 15 to 20). PA1 1. A connect state for the PLC in which the PLC is connected to the telephone line but in which the PLC is not responsive to other inputs; PA1 2. A ring decoder which detects either divided coded ringing signals or divided ringing signals in a novel manner; PA1 3. Means for testing whether a revertive call has been properly terminated; PA1 4. A low power switch so that the PLC can run on telephone line power only without battery backup; and PA1 5. A limited current source for the telephone subscriber and which draws power from the telephone line for isolating the busy tone from the telephone subscriber.
The present invention (the "PLC") includes all of these features but provides them in an improved way together with other features The PLC utilizes a state machine of the Mealy type with peripheral circuitry. A Mealy state machine is a device whose output and next state are a function of both the present state and inputs. The state machine is embodied in a logic IC including a gate array in which the logic of the PLC is carried out. The peripherals include a line detector for detecting the busy status of a telephone line, a hook detector for detecting whether the telephone subscriber is off-hook, a ring detector for detecting the presence of ring signals on a telephone line, a tone generator for applying a busy tone to the subscriber station (and not the telephone line) and other circuitry to be described later in this patent.
The PLC also includes the following features not incorporated in the prior art.
These and other advantages of the present invention are described in greater detail in the description.